Today, both chemical and energy industries rely on petroleum as the principal source of carbon products and energy. However, petroleum energy is under-used because of the remote locations of methane reserves, the relatively high transportation costs, and the thermodynamic and kinetic stability of this energy and chemical resource. Currently, the conversion of natural gas in fuels and chemical products is a complex and expensive process that requires multiple stages, and the main industrial use of methane is in the production of synthesis gas via steam reforming, a highly endothermic process. Synthetic gas in turn is converted to methanol at elevated pressures. Finally, formaldehyde would be obtained by oxidation of methanol at high pressure.CH4+H2O→CO+3H2   First ProcessCO+2H2→CH3OH   Second ProcessCH3OH+½O2→HCHO+H2O   Third Process
The production of methanol is important because methanol can be used to produce other important chemicals such as olefins, acetic acetate, acetate esters, and polymer intermediates. Thus, a direct conversion of methane into methanol and formaldehyde would be highly attractive compared to the current process that is expensive and energy intensive with corresponding environmental impacts.
Selective oxidation of methane has been studied for over 35 years by individuals as well as academic and government researchers with no commercial success. The direct route for methane conversion has remained one of the major scientific and technological challenges, since the catalytic activity and selectivity to C1-oxygenates are still far from a possible industrial application.
The direct conversion of methane into valuable chemicals, i.e., CH3OH and HCHO, involves partial oxidation under fuel rich conditions, i.e., O2:CH4 molar ratio is below 0.5. The use of fuel-rich mixtures with an oxidant minimizes the extent of combustion reactions, which yield unwanted carbon oxides. Under these conditions, purely gas-phase oxidation reactions require high temperatures, which are detrimental for the control of selectivity of the desired products. Accordingly, considerable efforts have been made in the last fifteen years to develop active and selective catalysts and reactor configurations for the partial oxidation of methane. For example, some articles (Gesser et al., Catal. Today, 1998, 42, 183-189; Yarlagadda, et al., Ind. Eng. Chem. Res., 1988, 27, 252-256; and Hunter, et al., Appl. Catal., 1990, 57, 45-54) disclose that reactor inertness is the key ingredient for obtaining high methanol selectivity and that a glass-lined reactor gives the highest selectivity to methanol and formaldehyde. Thermodynamic and kinetic studies reveal that the rate-limiting step of the partial oxidation of methane is the first H-abstraction from the C—H bond. Thus, initiators and sensitizers have been examined in order to decrease the energy barrier of this H-abstraction.
Several authors (Han, et al., Chem. Lett., 1995, 24, 931-932; Bromly, et al., Comb. Sci. Technol., 1996, 115, 259-296; Otsuka, et al., Catal. Today, 1998, 45, 23-28; Otsuka, et al., J. Catal., 1999, 185, 182-191; Tabata, et al., J. Phys. Chem., 2000, 104, 2648-2654; Bañares, et al., Catal. Lett., 1998, 56, 149-153) claim that nitrogen oxides promote gas-phase reactions with methane. Based on thermodynamic considerations, in Bromly, et al., Comb. Sci. Technol., 1996, 115, 259-296; Otsuka, et al., Catal. Today, 1998, 45, 23-28, kinetic models for the CH4+NO+O2 reaction were developed. The predictions of these kinetic models afforded excellent descriptions of the experimental data, obtained at atmospheric pressure, over the entire range explored. In these contributions, it was claimed that CO is the oxidation product but in no case were HCHO or CH3OH recorded as oxidation products. Tabata et al., Appl. Catal., A: Gen., 2000, 190, 283 proposed a reaction model for the conversion of CH4 to CH3OH and HCHO using either NOx (x=1, 2), or NO2+O2 as oxidant agents. Specifically, the calculated transition barrier of H-abstraction from the CH4 of the reaction CH4+NO2→CH3+HNO2 was lower than that for the reaction CH4+O2→CH3+HO2. The decrease in the transition barrier was experimentally verified by the linear enhancement of CH4 conversion with the NO2 concentration in the CH4+O2+NO2 mixture, and the experimental results of selectivity to C1-oxygenates were satisfactorily described by using the calculated values of the transition barriers and rate constants of the selected reaction routes from the methoxide radical (CH3O) to CH3OH and HCHO.
Literature survey reveals that transition oxides of copper (Cu), vanadium (V), molybdenum (Mo), iron (Fe), cobalt (Co), and some multi-component catalysts supported on various carriers are generally used as catalysts for such partial oxidations in the gas phase (Barbero et al., Chem. Commun., 2002, 1184-1185; Tabata et al., Catal. Rev. Sci. Eng., 2002, 44, 1-58; and Navarro et al., Metal Oxides: Chemistry and Applications, 2006, 463-490, CRC Press, Fla., Boca Raton). Using ZSM-5 zeolite as a carrier and Fe3+ oxide as a redox oxide, methane is oxidized in the presence of N2O gas to produce methanol. Wood, et al., J. Catal., 2004, 225, 300-306 discloses the methanol formation reactions on Fe/A1-ZSM-5 via the oxidation of methane by nitrous oxide, with methanol selectivity less than 2% at reaction temperatures above 250° C. This work also claims that when H2O is introduced at these reaction temperatures, the rate of methanol formation from the surface methoxy species increases. Water was added to the catalyst after formation of surface radicals, which are generated with interaction of N2O and CH4 on a catalyst surface. Mesoporous VOx/SiO2 catalysts have been used with the high efficiency of mesoporous VOx/SiO2 catalysts for selective partial oxidation of methane to formaldehyde (Launay et al., J.Catal., 2007, 246, 390).
Obtained results verify the success of a reaction conducted by a homogenous process promoted by the use of a nitrogen oxide in a certain proportion, together with methane and an oxidizing agent (such as air, or oxygen diluted with an inert diluent). Applicants have also developed a process that achieves the selective oxidation of methane to C1-oxygenates without the use of a catalyst.